The present invention relates generally to digital modem devices, and more particularly to a high data rate digital modem for use in high frequency (HF) communications.
High frequency (HF) communications can occur by way of line-of-sight, ground wave, surface wave or skywave propagation. The dispersive nature of the skywave communication link make it the most difficult for use with high data rate transmission. Digital skywave modem communications devices for use in the HF band have been typically limited in terms of the data rate at which digital data may be transferred. HF communications involves the transmission and reception of digital data reflected from ionospheric layers which vary in their reflecting properties. In particular, different layers reflect with different efficiencies, and each respective layer introduces phase errors into the reflected data which are difficult, if not impossible, to predict or control. In addition there are problems of fading due to ionospheric changes, long delay multipaths, and symbol dispersion which cause interference that severely degrades communication performance.
Communication performance can be improved by employing channel adaptive equalization techniques. A number of adaptive equalization techniques have been proposed and some been implemented. However, adaptive equalization requires long training signals, and adaptation time is long, preventing the waveform to hop at a rapid rate. Such a relatively slow hopping communication technique is susceptible to frequency following interference that can cause severe degradation to communication link. High performance communication can also be achieved using the direct sequence pseudo-random noise (DSPN) technique, in which an information bit is modulated with a wideband chip stream. However, it has been determined that in order to compensate for the dispersion and fading due to intramodal multipath, a processing gain of about 40 dB is required. With such a gain requirement, a 9600 bit per second data rate would require a bandwidth of about 96 MHz, which far exceeds that achievable in HF skywave propagation, and would require a special design HF antenna that can support such a wide bandwidth. Consequently, there is a need for a more achievable technique to implement high speed HF communications.
A technique has been investigated for use with cellular communications, and is discussed in U.S. Pat. No. 4,189,677 entitled "Demodulator unit for Spread Spectrum Apparatus Utilized in a Cellular Mobile Communication System," and U.S. Pat. No. 4,222,115 entitled "Spread Spectrum Apparatus for Cellular Mobile Communication Systems." These patents discuss the adaptation of frequency hopped digital phase shift keying to the UHF band for use in digital cellular communications systems.